1. Field of the Invention
The present invention relates to a disk for use in a toroidal type continuously variable transmission which can be used in vehicles, various kinds of industrial machines, and the like.
2. Description of the Related Art
A toroidal type continuously variable transmission comprises, for example, as shown in FIG. 15, input and output disks a and b which are disposed concentrically with each other, and a power roller c which is interposed between the respective traction surfaces f and i of the input and output disks a and b1.
In the input disk a, between the small diameter end portion d and large diameter portion e thereof, there is formed a traction surface f the cross section of which provides a concave-arc shape and, in the output disk b1, similarly, between the small diameter end portion g and large diameter portion h thereof, there is formed a traction surface i the cross section of which provides a concave-arc shape. On the side of the input disk a that is distant from the power roller c, there is concentrically disposed a loading cam through a plurality of engaging rollers (both of which are not shown), so that, due to the oil pressure that is supplied between the loading cam and input disk a, a driving force proportional to a torque can be applied toward the input disk a.
The power roller c is a device which can be frictionally engaged with the respective traction surfaces f and i of the input and output disks a and b1 to thereby transmit power; and, the power roller c is supported by a trunnion j in such a manner that it can be inclined in the diameter direction of the input and output disks a and b1. And, if the trunnion j is operated by a drive mechanism (not shown) to thereby change the contact positions of the power roller c in the diameter direction thereof with respect to the input and output disks a and b1, then a rotation speed ratio between the input and output disks a and b1, that is, a speed change ratio can be varied continuously.
By the way, the toroidal type continuously variable transmission is required to transmit a higher torque and, for this reason, the input and output disks, a, b1 and power roller c receive very large repeated bending stress and repeated shearing stress when compared with ordinary mechanical parts (such as ordinary gears and bearings); and, in the input and output disks a and b1, especially, as shown by fine hatchings in FIG. 17, the traction surfaces f, i, small diameter end portions d, g, and inside diameter surfaces at the small diameter end portion side d (g) receive large repeated bending stress and repeated shearing stress. Therefore, when manufacturing the input and output disks a and b1, it is necessary to use such highly durable material that can resist such repeated bending stress and repeated shearing stress.
Conventionally, to manufacture the input and output disks a and b1, for example, as shown in FIG. 12, cylindrical-shaped material (carburized steel or the like) having a length equal to the axial length of the input and output disks a and b1 is shaved or cut to thereby produce such a final shape as shown in FIG. 16.
However, in the conventional method for manufacturing the input and output disks a and b1, the yield of the material is poor and it takes long time to cut or shave the material, with the result that the production costs of the input and output disks a and b1 are soaring.
Also, because a metal flow (the flow of structure) k is arranged along the axial direction of the disk, in the traction surfaces f and i with which the power roller c is frictionally engaged with a large pressure, the metal flow k comes to an end and, actually, does not extend along the traction surfaces f and i. As a result of this, not only the material is easy to peel off in the portions of the traction surfaces f and i with which the power roller c is frictionally engaged, but also an impact crack or a fatigue crack is easy to occur in the input and output disks a and b starting at the broken portions of the metal flow k, thereby providing an obstacle to the long lives of the input and output disks a and b1.